Developmental Biology: by 8oCYZ393


                     the emergence of biological form

"development of an individual," 1872, coined from Gk. on (gen. ontos) "being" (prp. of einai "to
be") + -geneia "origin," from -genes "born."

         Genotype                   Development                    Phenotype
Preformationism: the homunculus

        Development consisted of an unfolding
        of a preformed, miniature adult. All that
        was required was the woman’s womb as
        an incubation chamber.
        what is one immediate theoretical
        problem with this idea?
             infinite regress
             An infinite regress in a series of propositions arises if
             the truth of proposition P1 requires the support of
             proposition P2, and for any proposition in the series Pn,
             the truth of Pn requires the support of the truth of Pn+1.
             There would never be adequate support for P1, because
             the infinite sequence needed to provide such support
             could not be completed.
            What is
embryos and more: the life cycle concept.
Formation of eggs and sperm
Self explanatory, but lots going
Cleavage and cleavage
                 Mitosis without
      Axis and cell fate specification
  Which end is up? How do cells know what to
become? What genes are involved? How do cells
       communication with each other?
Cell lineage and fate maps
     What they are and how they are
How to make a gut and a multilayered embryo
Formation of the nervous system, with
  consideration of the neural crest
Formation of organs from rudiments: eyes, limbs
    Differentiation and Stem
The end game: making the cells that do the work.
    Differentiation and Stem
The end game: making the cells that do the work.
     And stem cells: the future of medicine?
  Indirect life cycles & metamorphosis
Complex life cycles means having two somewhat
 separate developmental programs that must be
          linked by a metamorphosis.
Special topics:
Regeneration, Apoptosis
           preformationism vs. epigenesis
                epigenesis = upon formation
    • on theoretical grounds, epigenesis was more like
another way to think about epigenesis is the
progression from a less to a more differentiated state
 •    the “cell theory” meant that all living things were composed
      of cells and that cells could only arise by division of already
      existing cells. Argued against preformationism.
 •    among sexually reproducing organisms, germ cells are the
      only ones that give rise to new organisms. In the 1840s, it
      was recognized that the egg itself is a single specialized
•    In 1876, using the sea urchin Toxopneustes lividus Hertwig
     observed a sperm entering into an egg during fertilization.
• how to make eggs and
• egg and sperm unite!
• many biochemical reactions happen:
  acrosome reaction, slow/fast block to
  polyspermy (why is polysperm bad?),
  change to membrane properties,
  initiation of cortical rotations etc.
• sperm:egg interactions and bindin
  (perhaps save for evo-devo?)
• meiotic reduction and polar bodies.
• cleavage is the mechanism by which a
    Cleavage: divide and
  single cell returns to a multicellular
  organism.    mobilize
• embryonic cleavage is different than
  regular mitosis in that it is rapid,
  involves no cell growth and usually
  omits the gap phases. Lots of different
  critters cleave in different ways: go over
  the main types of cleavage
• cleavage is not just about dividing up
  the embryo: cleavage planes are the
  principal mechanisms by which
  maternal determinants are sequestered
  into particular lineages.
• The egg is a highly specialized cell with
  anisotropically distributed
• it is the unequal distribution of these
  determinants that are responsible for
  cell autonomous fate specification and
  unequal cell divisions.
• what are they? RNA/protein
• use the bicoid example: start with flies,
  move to squirts (whittakers
• move to the concept of the organizer.
      Cell fate specification/axis
• axis formation is the formation fo the
  basic body features and where they will
• specification is a transcriptional state
  that a cell enters into: cell autonomous
  or conditional. only two types.
• fate of specified cells can be altered
  experimentally. difference between
  specified and committed or determined.
• Hox genes.
    Cell lineage/fate

• why there are useful and how they are
• general labelling techniques and how
  they are used.
• a central concept: intercellular
Induction/pattern formation
• can happen in three ways: diffusion
  (morphogen); direct contact
  (transmembrane proteins) or gap
  junctions (coupling).
• induction causes new gene
  transcription and change in the
  development of the induced cell.
• permissive vs. instructive inductions
  and the issue of competence.
• lateral inhibition
• the concept of germ layers
• the formation of the gut, all embryos go
  through this stage
• It is neither birth, marriage nor death,
  but gastrulation which is truly the most
  important time of your life. many things
  are happening.
• maternal/zygotic transitions.
• talk about different gastrulation
  mechanisms (tissue level) as well as
  the cytoskeleton.
• Formation of the nervous system in
• neural crest as a major innovation.
• neurocristopathies.
• synapse formation averts the death
• epithelial mesenchymal interactions as
  the fundamental basis of
• select a couple of models like limb
  formation, heart formation, eye
  formation, tooth formation where there
  are multiple tissue interactions.
• what makes a differentiated cell? Really
 Differentiation/Stem cells
 a continuation of embryogenesis, but
  irreversible restriction of developmental
  potential (except in the case of
• models of cell differentiation (with
  respect to locking in patterns of gene
• differentiated cell is characterized by
  the proteins it contains, and
  consequently the function in performs
  (could reverse these)
• use the myoD example for fibroblasts.
   Germ cells: where do they come
• insert pic of germ/somatic cell
• different cells do it differently: germ line
  specification (vasa) and early segregation
  vs. genital ridge and inductive
  Indirect life cycles

• metamorphosis in frogs/flies/sea
• the role of hormones in co-ordinating
  multiple developmental events.
      Special topics:

• regeneration.
• apoptosis.
• stem cell cloning technologies.

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